1. Field of the Invention
The present invention relates to a NiSiCr-alloyed heat-resistant cast iron composition that has an austenitic matrix and nodular graphite in the microstructure. The composition exhibits excellent oxidation resistance and mechanical properties at elevated temperatures. The composition is suitable for components exposed to high temperatures and mechanical loading, especially those components in automobile engine systems such as exhaust manifolds, turbocharger housings, and catalytic converter housings.
2. Description of the Prior Art
There are currently, sorted by matrix structure, two types of heat-resistant cast irons used for engine exhaust components, ferritic and austenitic. Alloyed ferritic cast irons with nodular or vermicular graphite, of which silicon-molybdenum alloyed cast irons are most widely used, exhibit good oxidation resistance and mechanical properties at high temperature. As automobile engine exhaust temperature trends upward, more austenitic heat-resistant cast irons are employed when the temperature is so high that ferritic cast irons cannot meet the requirements in oxidation resistance and mechanical properties, especially yield strength and ultimate tensile strength.
The worldwide most frequently used austenitic cast iron in engine exhaust applications is Ni-Resist D5S in ASTM A439. This material is a high-alloyed nodular graphite cast iron comprising by weight less than 2.3% C, 4.9-5.5% Si, less than 1.0% Mn, 34-37% Ni and 1.75-2.25% Cr, with a minimum elongation of 10%, a minimum yield strength of 207 MPa and a minimum ultimate tensile strength of 449 MPa at room temperature. This material provides excellent oxidation resistance and superior yield strength and ultimate tensile strength over ferritic cast irons at high temperature. However, this is an expensive solution because the material has a very high nickel content.
Within the public domain, other documents exist regarding austenitic cast irons than Ni-Resist D5S for high temperature applications such as D4 in ASTM A439 comprising by weight 28-32% Ni, 4.5-5.5% Cr and 5-6% Si, and D4A in ASTM A439 comprising by weight 29-32% Ni, 1.5-2.5% Cr and 4-6% Si. The former has sufficient oxidation resistance, but does not have sufficient elongation because of the high Cr content that forms continuous interdendritic carbides. The latter has similar mechanical properties to Ni-Resist D5S, but does not have adequate oxidation resistance. Besides, these materials all have relatively high nickel content that leads to a higher cost solution.
Patent publication U.S.2006/0191604 discloses an austenitic heat-resistant spheroidal graphite cast iron comprising by weight 1-3.5% of C, 1-6.5% of Si, 3% or less of Cr, 10-40% of Ni, 1-4.5% of Mo, and 0.001-0.5% of Sn and/or Sb as (2Sn+Sb) and 0.1% or less of graphite-spheriodizing element. This material achieves good oxidation resistance and good yield strength by adding a large amount of expensive element Mo and keeping nickel and silicon contents at a high level as shown in the presented examples that contain 1.18-4.49% of Mo, 26.9-35.9% of Ni and 3.75-5.13% of Si. This makes the material less economically attractive. Also, the room temperature elongation of the described examples in this publication ranges from 2.1-5.3%, which is significantly lower than that of Ni-Resist D5S and cannot meet the specifications for most automobile engine exhaust components.
There are publications describing austenitic heat-resistant cast irons with lower nickel content such as U.S. Pat. No. 4,528,045 that discloses a spheroidal graphite cast iron comprising 18-24% of Ni, 3-5% of Cr and 3.5-6% of Si by weight. This material has higher oxidation resistance than Ni-Resist D5S, but does not have sufficient room temperature elongation because of the high chromium content.
Another material option for engine exhaust components is using high-alloyed heat-resistance cast steels. Some of the austenitic steels can provide better oxidation resistance and mechanical properties than Ni-Resist at both room and elevated temperatures. However, these steels have much higher melting points than cast iron and may have poor castability, which leads to high energy consumption and makes the production process more complicated and expensive. Consequently, the process costs of these cast steels will be inevitably higher than that of austenitic cast irons.
Accordingly, the object of the present invention is to provide a lower cost austenitic heat-resistant cast iron that possesses similar or improved oxidation resistance, yield and ultimate tensile strengths and elongation to those of Ni-Resist D5S at room and high temperatures. This composition would be a substitute for Ni-Resist D5S in engine systems.